An assembled battery configured by coupling a plurality of secondary batteries (unit batteries) in series is mounted in a hybrid vehicle or an electric vehicle. For this kind of assembled battery, it is necessary to individually detect the voltage of each secondary battery for the purposes of calculating the capacitance, and a conservation and management, of each secondary battery. However, because the number of series connections of the secondary batteries configuring the assembled battery is very large in the above-described application, the potential of the secondary batteries increases in accordance with a connection position in the assembled battery, and a high voltage is applied to a secondary battery voltage detection device.
JP-A-2008-145180 (corresponding to US 2008/0150516 A) discloses a voltage detection circuit including an operational amplifier, a first capacitor, one end of which is coupled to an inverting input terminal of the operational amplifier, a second capacitor and a discharge circuit, which are coupled between the inverting input terminal and an output terminal of the operational amplifier, and switches coupled one between each of terminals of unit batteries and the other end of the first capacitor. A switch between a plus terminal of a unit battery and the first capacitor, and the discharge circuit, are turned on, thus charging the first capacitor, and subsequently, in a condition in which the discharge circuit is turned off, a switch between a minus terminal of the unit battery and the first capacitor is turned on in place of the above-described switch, thus detecting the voltage of the unit battery.
A digital isolator wherein a differential driver and a differential amplifier are combined by a first capacitor is disclosed in U.S. Pat. No. 4,748,419.